On September 24th, 2020, at the Powdermill Nature Reserve in Rector Pennsylvania, researchers from the Carnegie Museum of Natural History caught one unique bird that’s genetically part male and part female, split down the middle and displaying the vibrantly colored features of both sexes.
This odd little bird belongs to a species known as the rose-breasted grosbeak (Pheucticus ludovicianus), a large seed-eating member of the cardinal family found all over North America. The male of this species is known for its opulently colored feathers, but this particular bird has uniquely distinctive colored feathers that differ on each side: the right side is rosy red like a male, while the left is brown-orange like a female.
This is because the animal is a rare example of bilateral gynandromorphism, where an animals’ external appearance is split down the middle by sex, half-male, and half-female.
You’re must be wondering, how on earth did this happen?
Well, sex determination in birds is a little bit different from us humans. In humans, the females have two copies of the same sex chromosome (XX) and the males have a copy of each (XY), but it’s essentially the opposite away around in birds. The males have a double sex chromosome (ZZ) and the females have one of each (ZW).
“The entire banding team was very excited to see such a rarity up close, and are riding the high of this once-in-a-lifetime experience. One of them described it as ‘seeing a unicorn’ and another described the adrenaline rush of seeing something so remarkable,” Annie Lindsay, bird banding program manager at Powdermill, said. “Bilateral gynandromorphism, while very uncommon, is normal and provides an excellent example of a fascinating genetic process that few people ever encounter.”
Gynandromorphy is believed to occur for various reasons depending on the species, for birds, it’s believed to occur when an egg accidentally develops with two nuclei, one carrying a Z and the other a W. If that egg is fertilized by sperm carrying two male Z chromosomes, the egg develops with both ZZ (male) and ZW (female) chromosomes.
The Researchers who discovered this bird is now interested to see whether it can successfully breed. Since only the left ovary is typically functional in birds and the left side of this bird is the female side, it’s theoretically possible for the individual to breed with a male. However, there’s a chance its unusual feathers might cause a territorial response from other males, which would dampen its chances of successfully mating.